Recovery of oil by in situ combustion



1951 J. c. TRANTHAM ETAL 3,007,521

RECOVERY OF OIL BY IN SI'FU COMBUSTION Filed Oct. 28, 1957 I OIL FUELGAS as on.

AIR 0R FUELGAS AIR OR 0 AIR OR 0 n v I INVENTORS J.C. TRANTHAM H.W. PARKER A T TORNEYS Unite S This invention relates to a process for initiating in situ combustion in a stratum containing hydrocarbon material and producing hydrocarbons therefrom.

The recovery of oil from underground strata containing the same by in situ combustion techniques has recently come into prominence in the petroleum industry. One of the problems in this type of operation is in the initiation and establishment of in situ combustion of the in-place oil to such an extent that the combustion can be driven into the formation by merely feeding air to the combustion zone. In accordance with one technique a'mixture of fuel gas and air is burned in a borehole in the hydrocarbon-containing stratum so as to heat up the adjacent formation. After a substantial burning period, the fuel gas content of the mixture is reduced and finally eliminated so that the combustion is driven into the formation and feeds solely on the air injected into the borehole. In the application of this technique to various formations the initial burning period is in ordinately extended and often it is extremely difficult to maintain a sufiicient heat reservoir around the ignition borehole to permit extension of the combustion zone into the formation and driving of same through the formation to a series of surrounding boreholes. It is particularly ditlicult to utilize this technique successfully where the combustion zone or front is to be advanced laterally through the formation by inverse air injection through surrounding boreholes. When utilizing inverse air injection technique a substantial period of time is generally required for the passage of air from the injection boreholes to the ignition borehole and the heat reservoir established during the initial phase of the combustion process cools appreciably and frequently is at a temperature below combustion supporting temperature when the inverse air arrives.

Accordingly it is an object of the invention to provide an improved process for the production of hydrocarbons from a hydrocarbon-bearing stratum by in situ combustion. Another object is to provide an improved process for initiating in situ combustion in a hydrocarbon-bearing stratum. A further object is to provide a process for initiating in situ combustion in a hydrocarbonbearing stratum which establishes a substantial heat reservoir of high temperature which provides time for inverse air to reach the heated area before the temperature drops below ignition temperature. Other objects of the invention will become apparent upon consideration of the accompanying disclosure. A broad aspect of the invention comprises heating the wall of a borehole in a hydrocarbon-bearing stratum to combustion supporting temperature by any suitable means and then injecting substantially pure oxygen or oxygen-enriched air into the heated formation so as to promote rapid combustion of the in-place hydrocarbon and raise the temperature of the formation, thereby establishing a substantial heat reservoir at a temperature at least about 1,000 F. above combustion supporting temperature; and then injecting air into the hot stratum so as tocontinue the combustion of in-place hydrocarbons and drive the combustion zone or front through the formation.

Contacting the hot formation in this manner with oxygen-enriched air or with substantially pure oxygen raises tes atent the temperature thereof to the range of about 1500 to 2500 F., depending upon the oxygen concentration.

A minimum oxygen concentration of is desired. Heating the formation to the temperature of 1500 to 2500 F. over a substantial section of the stratum around the ignition borehole builds up a sufficient heat reservoir to permit the flow of inverse air from surrounding boreholes used as injection points before the temperature of the reservoir drops below ignition temperature, which is usually in the range of 500 to 600 F. when using air as the oxidizing medium.

In a preferred embodiment of the invention the wall of the ignition borehole is heated up by injecting a mixture of fuel gas and air into the stratum at one or more points spaced radially a few feet, such as l to 10 feet, from the ignition borehole and burning the combustible mixture on the walls of the borehole so as to heat same to combustion supporting temperature of the in-place hydrocarbon in an oxygen containing ambient of at least 30% oxygen concentration. When the wall of the borehole has reached a temperature of at least500 F., the flow of the combustible mixture is terminated and the heated stratum adjacent the borehole is contacted with pure 0 or O -enriched air so as to initiate combustion of the hydrocarbon in the formation and raise the temperature of the formation to at least 1500 F, whereupon the concentration of O in the combustion supporting gas is decreased to that of air, either gradually or suddenly, so that air is then utilized as the combustion supporting gas. It is preferred to inject the oxygen or oxygen-enriched airin the same manner as the mixture of fuel gas and air, i.e., through the formation from one or more boreholes spaced a short distance from the ignition borehole; however, the injection of oxygen or oxygen-enriched air through the ignition bore-hole may be utilized to establish the heat reservoir necessary to facilitate the driving of the combustion zone through the formation by injection of air either directly through the ignition borehole or from boreholes spaced apart from the ignition borehole.

After the formation around the ignition borehole has been heated up by the injection of oxygen or oxygenenriched air, the flow of this combustion supporting gas is terminated and air is injected through a plurality of boreholes surrounding the ignition borehole, and when the injection air arrives at the combustion area the combustion of in situ hydrocarbon is reestablished since the heated area is still at combustion supporting temperatures. The inverse injection of air may be eifected through the boreholes through which the combustible mixture is injected or it may be injected through boreholes spaced farther from the ignition borehole, as in a typical S-spot well pattern. It is also feasible to first inject air through the boreholes spaced close to the ignition borehole, i.e.,

those through which the combustible mixture is injected, and then close off these boreholes and inject air through the more remote boreholes.

A more complete understanding of the invention may be obtained upon consideration of the accompanying drawing of which FIGURE 1 is a plan view of boreholes spaced around an ignition borehole in accordance with the invention; and FIGURE 2 is an elevation through a formation showing the arrangement of boreholes and equipment for effecting the process of the invention.

Referring to FIGURE 1, an ignition borehole 10 is surrounded by a series of injection boreholes 12 spaced from about 1 to 10 feet from the ignition borehole. Boreholes 1 are spaced considerably farther from borehole formation 16 and is provided with a casing 18 and a tubing string 20 for the recovery of oil when utilizing inverse air injection. Conduit 22 provides means for introducing air, oxygen, oxygen-enriched air, and fuel gas. Borehole 12 is provided with conduit means 24 which connects with a tubing string 26 for the introduction of fuel gas through conduit 28 and air and oxygen through conduit 30. Borehole 14 is provided with a tubing string 32 for recovery of oil when utilizing direct air injection and with conduit 34 for inverse injection of air or other oxygen-containing gas.

Typical operation comprises injecting a combustible (preferably stoichiometric) mixture of fuel gas and air through conduits 28 and 30, respectively, into conduit 24 so that the mixture is delivered to borehole 12 via conduit 26 and to stratum 16. Boreholes 14 are sealed off by closing the valves in the conduits connecting with the casinghead so that the combustible mixture passes through the short space in stratum 16 between boreholes 12 and borehole 10. The mixture is ignited in borehole 10 by any suitable means, such as an electric spark or squib or other igniter, and the flow of the iixture is adjusted so that the combustion takes place on the Wall of the formation within the borehole. Several days or even several weeks of this initial phase of the process may be required to heat the wall of the borehole to a temperature of at least 500 F., because of the loss of heat to the surrounding formation and the relatively high heat capacity thereof.

When suitable temperature in the formation around the borehole 10 is reached, the flow of combustible gas through conduit 26 is shut off and pure or O -enriched air is passed through conduits 30, 24, and 26 so that rapid burning of hydrocarbon in the formation around the wall of the borehole takes place and the temperature of the section of the stratum adjacent the borehole is raised to the range of 1500 to- 2500 F. Generally, a period in the range of hours to several days of oxygen injection heats up a suflicient section of the stratum adjacent ignition borehole 10 to provide ample time for passing inverse air through the formation either from boreholes 14 or 12 before the temperature of the heated section drops below combustion supporting temperature in the presence of the inverse air. By first injecting in verse air through borehole 12 until the combustion zone reaches the vicinity of this borehole and then sealing boreholes 12 and injecting air through boreholes 14, the inverse air injection technique moves the combustion front through the formation until it reaches the outer ring of boreholes. Injection of air first thru boreholes 12 consumes minimum time for passage of inverse air to the combustion zone.

It is also feasible to move the combustion front through the stratum 16 by injection of air through conduit 22 to borehole 10 in strata of a type in which direct air injection does not build up a wall of heavy hydrocarbon beyond the advancing combustion zone which blocks off the flow of gas through the formation.

Of course it is to be understood that the objective of in situ combustion is to produce hydrocarbons from the formation in which the operation takes place. When utilizing the preferred method of inverse air injection the oil driven from the formation passes to borehole 10- from which it is recovered through production string 20 by conventional means. Because of the intense heat in borehole 10 during the injection of pure oxygen or oxygen-enriched air, it is advisable to lower production string 20 into the borehole after the high temperature phase of the process and after the combustion zone has moved into the formation sufficiently that the wall of the borehole has cooled down to a temperature which is not injurious to the production string. In applications of the invention which utilize direct air injection through the ignition borehole, oil and other hydrocarbons are produced through production string 32 in boreholes 14.

It is to be understood that the initial heating of the stratum 16 around ignition borehole 10 may be effected by any suitable means, such as by an electric or gas heater positioned adjacent the stratum, after which the formation is contacted with pure oxygen or oxygen-enriched air in accordance with the invention described herein.

A number of tests have been performed to find methods for the ignition of tar sand formations which are practicable in the field. The oxygen slug technique of this invention has been the most successful thus far. First premixed air and propane in substantially stoichiometric proportions were injected into the tar sand and the flow was adjusted to burn the mixture on the surface thereof. After a period of preheating by this means, the air-propane mixture was replaced with pure oxygen for a brief time interval. resulted in intense burning on and beneath the surface of the tar sand during which time the tar sand became red hot. The oxygen was then replaced by air, alone, and the combustion front propagated through the sand.

In these tests, tar sand cores were used which were 4 /2 long sections of 2 /2 diameter. The individual cores were mounted in a section of 2%" ID. brass tubing by means of talc packed around the core. Provision was made at the bottom of the tube to admit either oxygen, air, or premixed air and propane. The top end of the core was exposed and premixed propane and air injected at the bottom end was burned as close as possible to the top surface of the core. The core and its holder were heated for about one hour by burning the air and propane mixture before the oxygen was injected. Oxygen alone flowed from 70 to 220 seconds at about the same rate as that used for the air in each test. When the oxygen was first applied, flames leaped 2 or 3 inches above the core and then sank into the sand surface and the surface of the core became red hot. At this point the flow of oxygen was cut off and air was injected and the combustion continued completely through the sand.

When air rates of 700 to 1,000 s.c.f.h./ft. were employed, the cores burned through by inverse injection and then back to the end of initial burning as a thermal echo or by reflection burning. The resulting cores were very fragile and light gray in color. There were many cracks throughout the cores and the sand was substantially totally depleted of hydrocarbon. In cases where air rates of 400 to 500 s.c.f.h./ft. were used, the core burned through from one end to the other but no thermal echo occurred and these cores were black from coke remaining on them.

Thermal echo or reflection burning when utilizing the inverse air injection technique is disclosed and claimed in application, Serial No. 529,916, filed August 22, 1955.

While the invention has been described in certain preferred embodiments, it will be appreciated that variations beyond the exact details set forth herein are possible without departing from the invention in its broadest aspects.

We claim:

1. A process for initiating in situ combustion in a hydrocarbon-bearing stratum which comprises injecting a combustible mixture of fuel gas and air into said stratum thru at least one injection borehole therein spaced from an ignition borehole therein a distance in the range of 1 to 10 feet so as to pass said mixture thru and permeate a substantial portion of the annular section of stratum adjacent said ignition borehole; igniting and burning said mixture on and in said annular section to heat same to combustion supporting temperature; while said section is at said temperature, contacting same with a free-oxygen-containing gas of at least 30% 0 concentration so as to ignite in-place hydrocarbon and establish an in situ combustion zone in said stratum having a temperature in the range of 1500 to 2500 F., thereby providing a high temperature heat reservoir for permitting flow of inverse air thru the stratum to the combustion zone from at least one remote borehole in said stratum spaced farther from said ignition borehole than first said injection borehole, before the temperature of said zone drops below combustion-temperature; and injecting air thru said at least one remote injection borehole so as to continue in situ combustion in said stratum.

2. The process of claim 1 wherein injection of said mixture is terminated after said combustion supporting temperature is reached and said oxygen-containing gas consists essentially of O and is injected thru said stratum from a plurality of injection boreholes radially outside of said ignition borehole a distance in the range of 1 to 10 feet to establish in situ combustion in said stratum.

3. A process for initiating in situ combustion in a hydrocarbon-bearing stratum which comprises injecting a combustible mixture of fuel gas and oxygen into said stratum thru a plurality of boreholes therein spaced around an ignition borehole in the range of 1 to 10 feet therefrom so as to cause said mixture to flow thru and permeate an annular forming the wall of said ignition borehole; igniting and burning said mixture on said wall so as to heat said annulus to a temperature of at least 500 F.; while said annulus is at said temperature, discontinuing the flow of said mixture and contacting said annulus with an oxidizing gas containing at least 30 percent free oxygen so as to initiate in situ combustion of said hydrocarbon; and after a substantial heat reservoir at a temperature of 1500 to 2500 F. has been established in said annulus, passing air to the combustion area so as to continue in situ combustion in said stratum.

4. The process of claim 3 wherein said air is passed to the combustion area thru said formation from a plurality of boreholes surrounding said ignition borehole and spaced iarther from. same than first said plurality of boreholes; and a mixture of combustion gas and produced hydrocarbons is recovered from said ignition borehole.

5. The process of claim 3 wherein said oxidizing gas consists essentially of O 6. The process of claim 3 wherein air is injected into said stratum thru said igition borehole to drive the combustionzone radially outwardly and a mixture of combustion gas and produced hydrocarbons is recovered from boreholes surrounding said igition borehole.

References Cited in the file of this patent UNITED STATES PATENTS 2,642,943 Smith June 23, 1953 2,793,696 Morse May 28, 1957 2,818,117 Koch Dec. 31, 1957 2,880,803 Parker Apr. 7, 1959 

